A ball screw equipped with a screw shaft, a nut and balls disposed between the screw shaft and the nut so as to allow their roll motions is able to reduce a friction coefficient in rotating the screw shaft to the nut, compared to a sliding contact type screw. Hence, the ball screw is put into practical use in various mechanical fields such as positioning mechanisms and feeding mechanisms of machine tools and robots and steering gears of automobiles.
In recent years, to raise an amount of allowable load, a roller screw equipped with rollers, as rolling members, instead of balls has been invented as disclosed in, for example, patent reference 1. In this roller screw, a screw shaft is provided to have an outer circumferential surface on which a roller rolling groove is formed, and a nut is provided to have an inner circumferential surface on which a spiral loaded roller rolling groove is formed to face the roller rolling groove of the screw shaft. There is formed a loaded roller rolling path between the roller rolling groove of the screw shaft and the loaded roller rolling groove of the nut and a plurality of rollers serving as rolling members are disposed on the loaded roller rolling path for accommodation therein. In the nut, a non-loaded roller return path is formed to connect one end of the loaded roller rolling path to the other end thereof. This non-loaded roller return path enables circulation of the rollers rolling on along the loaded roller rolling path.
FIG. 26 shows a retainer 51 assembled in a roller screw proposed by the present applicant. This retainer 51 is arranged between, of a plurality of rollers, each adjacent two rollers to prevent the two rollers from contacting each other. On each end of the retainer 51 in the moving direction thereof, there is formed a concave surface 53 of which a contour is inconformity with the outer circumferential surface of each roller.
FIGS. 27(A) and 27(b) each shows a chain of rollers with such retainers 51 assembled with the rollers. Rollers 52 are cross-arranged in such a manner that axial lines of adjacent two of the rollers 52 are perpendicular to each other when being viewed along the moving direction of the rollers 52.